1. Field of the Disclosure
The present invention relates to an apparatus for drilling a subterranean bore hole, particularly but not exclusively an oil, gas or geothermal well, using a technique known as managed pressure drilling.
2. Description of Related Art
The drilling of a borehole or well is typically carried out using a steel pipe known as a drill string with a drill bit on the lowermost end. The entire drill string may be rotated using an over-ground drilling motor, or the drill bit may be rotated independently of the drill string using a fluid powered motor or motors mounted in the drill string just above the drill bit. As drilling progresses, a flow of mud is used to carry the debris created by the drilling process out of the borehole. Mud is pumped through an inlet line down the drill string to pass through the drill bit, and returns to the surface via the annular space between the outer diameter of the drill string and the borehole (generally referred to as the annulus). Mud is a very broad drilling term, and in this context it is used to describe any fluid or fluid mixture used during drilling and covers a broad spectrum from air, nitrogen, misted fluids in air or nitrogen, foamed fluids with air or nitrogen, aerated or nitrified fluids to heavily weighted mixtures of oil or water with solid particles. Significant pressure is required to drive the mud along this flow path, and to achieve this, the mud is typically pumped into the drill string using one or more positive displacement pumps which are connected to the drill string via a pipe and manifold known as the standpipe manifold.
The geological formations into which such boreholes are typically drilled often comprise a reservoir of pressurised fluid (oil, gas and/or water), and the mud flow, in addition to flushing out the debris and cooling the drill bit, pressurises the borehole, thus substantially preventing uncontrolled flow of fluid from the formation into the borehole. Flow of formation fluid into the borehole is known as a kick, and, if not controlled, can lead to a blow out. Whilst pressurising the borehole is required to avoid kicks or a blow out, if the fluid pressure in the borehole is too high, the fluid pressure could cause the formation to fracture, and/or mud could penetrate and be lost to the formation. Thus, whilst the pressure provided by the weight of the mud in the bore hole, and the dynamic pressure created by the pumping of the mud into the borehole may be enough to contain the fluid in the formation, for many formations greater and faster control over the fluid pressure in the borehole is required, and one drilling method suitable for drilling into such formations is managed pressure drilling (MPD).
Managed pressure drilling (MPD) involves controlling the bottom hole pressure by the application of a back-pressure to mud exiting from the annulus of the borehole. The most relevant elements of a conventional prior art managed pressure drilling system are illustrated schematically in FIG. 1. This figure shows a borehole 10′ which extends into a geological formation 11′ comprising a reservoir of fluid such as oil, gas or water. A drill string 12′ extends down into the bore hole 12′. At the lowermost end of the drill string 12′ there is a bottom hole assembly (BHA) 14′ comprising a drill bit, a mud motor, various sensors, and telecommunications equipment for transmitting readings from the sensors to surface monitoring and control equipment. The uppermost end of the drill string 12′ extends to a drilling rig (not shown for clarity).
The borehole 10′ is capped with a well head 18′, and a closure device 20′ such as a rotating blow out preventer (BOP) or rotating control device (RCD). The drill string 12′ extends through the well head 18 and closure device 20′, the closure device 20′ having seals which close around the exterior of the drill string 12′ to provide a substantially fluid tight seal around the drill string 12′ whilst allowing the drill string to rotate about its longitudinal axis, and to be reciprocated into and out of the borehole 10′. Together, the well head 18′ and closure device 20′ isolate the fluid in the annulus 16′.
In this example, the drill string 12′ extends from the closure device 20′ to a driving apparatus 22′ such as a top drive, and the uppermost end of the drill string 12′ is connected to the outlet port of a standpipe manifold 24′ which has an inlet port connected by an inlet line to a mud pump 26′. The well head 18′ includes a side port 18a′ which is connected to an annulus return line 28′, and which provides an outlet for fluid from the annulus 16′. The annulus return line 28′ extends to a mud reservoir 34′ via an adjustable choke or valve 30′ and a Coriolis flow meter 32′ which is downstream of the choke/valve 30′. Filters and/or shakers (not shown) are generally provided to remove particulate matter such as drill cuttings from the mud prior to its return to the mud reservoir 34′.
During drilling, the top drive 22′ rotates the drill string 12′ about its longitudinal axis so that the drill bit cuts into the formation, and the pump 26′ is operated to pump mud from the reservoir 34′ to the standpipe manifold 24′ and into the drill string 12′ where it flows into the annulus 16′ via the BHA 14′. The mud and drill cuttings flow up the annulus 16′ to the well head 18′, and into the annulus return line 28′, and the adjustable choke or valve 32′ is operated to restrict flow of this fluid along the annulus return line 28′, and, therefore, to apply a back-pressure is applied to the annulus 16′. This back-pressure is increased until the fluid pressure at the bottom of the wellbore 10′ (the bottom hole pressure) is deemed sufficient to contain the formation fluids in the formation 11′ whilst minimising the risk of fracturing the formation or causing mud to penetrate the formation. The rate of flow of fluid out of the annulus 16′ is monitored using the flow meter 32′, and compared with the rate of fluid into the drill string 12′, and this data may be used to detect a kick or loss of mud to the formation.
Such a system is, for example, disclosed in U.S. Pat. No. 6,575,244, and U.S. Pat. No. 7,044,237.
Managed pressure drilling systems in which a pump is provided to assist in the development of the required bottom hole pressure by pumping mud back into the annulus 16 via the annulus return line are also known and are, for example, disclosed in U.S. Pat. No. 7,185,719, U.S. Pat. No. 7,395,878, US 2007/0151762, WO 2007/081711, and WO 2008/051978.